The invention relates generally to thermal barrier coating systems, in particular, thermal barrier coating systems exposed to high temperatures, such as in a gas turbine engine.
Higher operating temperatures for gas turbine engines have been continuously sought in the art in order to improve the operating efficiency of the engine. However, as operating temperatures are raised, the high temperature capabilities of the components in the engine must also increase. To this end, various nickel-base and cobalt-base superalloys have been employed, which incorporate oxidation-resistant and corrosion-resistant overlay and diffusion-type coatings.
Further improvements in the high temperature capabilities of components have been realized by coating engine components with a thermal barrier coating (TBC), in addition to the overlay and diffusion-type coatings mentioned above. TBCs are generally formed of ceramic materials, such as zirconia (ZrO.sub.2) stabilized by an oxide material. To promote adhesion between the thermal barrier coating and the underlying substrate, bond coats are utilized. One type of bond coat is formed of MCrAlY, wherein M is chosen from the group consisting of iron, cobalt, nickel and combinations thereof.
Such bond coats may be deposited by various techniques, including electron beam physical vapor deposition (EBPVD), as well as plasma spray techniques, including low pressure plasma spray (LPPS), and air plasma spray (APS). Of these bond coats, APS bond coats have been used for their ease of deposition.
The present inventors have recognized deficiencies with thermal barrier coating systems including air plasma sprayed bond coats. Particularly, the air plasma sprayed bond coat may have insufficient roughness to promote good adhesion of the thermal barrier coating thereto. In addition, the TBC system has tended to fail at the interface between the TBC and the bond coat, due to propagation of cracks along interface.
One technique that addresses the surface roughness of the bond coat is U.S. Pat. No. 5,817,372, to Zheng, commonly owned by the present assignee. The disclosed technique deposits a bond coat utilizing vacuum plasma spraying (VPS) technique or high velocity oxy-fuel (HVOF) technique. Two metal powders are utilized for deposition, including one having a fine particle size distribution, and another having a coarser particle size distribution. The fine particles preferentially melt during the deposition process. Upon solidification, the coarser particles are bonded to the substrate.
It would be desirable in the art to provide a thermal barrier coating system that has improved durability and robustness, while providing improved resistance to oxidation and corrosion at high temperatures.